This invention relates to an engine and a method of operating an engine, and in particular to a method of operating an internal combustion reciprocating piston engine. The invention also relates to a method of operating a reciprocating piston machine, which may take the form of an engine or a compressor.
Since its conception the design and operation of the internal combustion engine has been subject to continuous development and improvement, with the result that the performance and emissions from such engines have improved dramatically. In recent years, efforts have focused on the aim of reducing undesirable engine emissions, such as the products of incomplete combustion (carbon monoxide (CO) and unburnt hydrocarbons (HC)), and oxides of nitrogen (NOx), which are recognised as having a significant impact on the environment and human health.
Recent developments have included improving combustion from inducing higher turbulence in the fuel air charge, direct injection to improve fuel dispersion, and experiments with ignition energy and disposition of the point or points of ignition in the combustion chamber. Piston and combustion chamber design have also received attention to produce swirl and squish effects. However, it has been shown that turbulence and swirl change the pattern and length of the flame front from the point of ignition and may result in uneven burning of the charge in the combustion chamber, and an even slower overall rate of combustion. Experiments have been carried out using earlier ignition of the charge to counter the slower overall burn resulting from the swirl effects, however this has been found in some cases to exacerbate the NOx output, although it may lower CO and HC levels.
One of the most significant recent developments was the xe2x80x9clean burnxe2x80x9d engine, with a view to reducing fuel consumption and reducing emissions of CO and HC. However, lean burn engines tend to produce relatively large amounts of NOx, due to the excess oxygen present at the high temperatures and pressures reached, particularly if the duration of combustion is extended due to early ignition of the charge.
It is among the objectives of embodiments of the present invention to obviate or mitigate one or more of these disadvantages. In particular, it is an object of embodiments of the present invention to obviate or mitigate one or more of the disadvantages inherent in conventional engine design and thereby allow improvements in the combustion process, and further to facilitate adaptation of the performance characteristics of an engine to suit a particular application.
According to one aspect of the present invention there is provided a method of operating an internal combustion reciprocating piston engine, the method comprising the steps of:
moving a piston within a chamber to compress a charge contained therein; and
igniting the compressed charge while the piston is being moved in the chamber at substantially constant or increasing velocity.
According to another aspect of the present invention there is provided an internal combustion engine in which a piston is reciprocally movable in a piston chamber to compress a charge which is ignited during a latter portion of a compression stroke, the engine comprising:
a rotating power output member; and
a connection between a piston and said power output member, characterised in that said connection includes means for moving the piston at a substantially constant or increasing velocity at the point of ignition.
The various aspects of the present invention will be primarily described herein with reference to four stroke spark ignited petrol engines comprising one or more cylinders, however aspects of the invention may also be applicable to engines utilising other fuels, such as natural gas, diesel oil and kerosene and engines operating on other cycles, such as the two stroke cycle, and compression ignition engines and engines utilising different ignition methods.
In conventional piston engines, each piston is directly connected to a rotating crankshaft by a piston rod. As a result, each piston moves harmonically and is travelling at maximum speed in mid-stroke. Thus, during the compression stroke, the piston accelerates from bottom dead centre (BDC), reaching maximum speed at mid stroke and thereafter decelerates at an increasing rate towards top dead centre (TDC). Ignition of the fuel/gas charge typically occurs between 25xc2x0 and 45xc2x0 before TDC, while the piston is decelerating from maximum speed, as dictated by the crankshaft piston connecting rod relationship. The relatively slow speed of the piston following ignition, up to and after TDC, results in the burning charge being maintained at high temperature and pressure for a relatively long period, thereby increasing the likelihood of the creation of undesirable combustion products, particularly NOx. In contrast, in the present invention, the piston is moving at a substantially constant or increasing velocity at the point of ignition. Although not wishing to be bound by theory, it is believed that the substantially constant or increasing velocity of the piston creates a positive and stable pressure gradient or pressure wave in front of the piston. The pressure wave will interact with the advancing flame front, increasing the flame speed and reflecting the flame back towards the roof of the combustion chamber, resulting in a faster overall combustion process, such that combustion of the charge occurs evenly and in a relatively short time interval. The ability to attain complete combustion in a shorter time interval allows the expansion or working stroke to commence earlier than has so far been practical, without the penalty of incomplete combustion. Thus, the combustion process is completed in conditions of lower turbulence and, therefore, more evenly and in minimum time, resulting in the production of minimum CO and HC components, and as the burning charge is maintained at high temperature and pressure for a shorter time the production of nitrous oxides is also minimised.
The mechanical configuration of the engine and in particular the configuration of the connecting means may take any suitable form, and may include an arrangement of cams and cranks, gears, cranks, eccentric drives and the like as will be apparent to those of skill in the art.
Preferably, the connection between the piston and the output member is arranged such that maximum torsional effect can be applied to the output member during an initial or earlier portion of the power or working stroke, when the pressure of the burning charge is at or near a maximum, and thus the output torque will be superior to a conventional engine. This may be enhanced by providing a relatively low piston descent rate following TDC, thereby allowing a more efficient utilisation of maximum heat release and, as a result, high cylinder pressure providing high torsional effort at the power output member.
Preferably, the piston speed is substantially constant or increasing at ignition of the charge.
Preferably also, the piston is moving at or around its maximum velocity when ignition is triggered.
According to another aspect of the present invention there is provided a reciprocating piston machine in which at least one of the length, duration and pattern of at least one piston stroke differs from the length, duration and pattern of another stroke.
In the case of a four stroke cycle all four strokes may differ in one or both of length and duration.
According to a further aspect of the present invention there is provided a four stroke reciprocating piston machine having a piston coupled to a rotating power output member, the four strokes corresponding to a 360xc2x0 rotation of the output member.
In accordance with embodiments of these aspects of the invention, the piston stoke lengths and velocities within the four cycles may be adjusted individually to satisfy differing heat release rates for various types of fuels, improve exhausting, and give better pumping efficiencies and thus higher volumetric efficiency. For example, by reducing the time span of the compression stroke it is possible to increase the rate of compression, which together with the higher piston speed at ignition, will assist in speeding up flame front movement, thereby reducing the overall time span of the complete combustion phase, where time, temperature and pressure have a significant influence on the production of oxides within the burning charge.
Preferably, at least one of the length and duration of the stroke of the expansion or power cycle is shorter than another stroke, and may be up to 50% shorter than another stroke. The duration of the expansion or power stroke may be reduced in proportion to the degree of rotation of the output member that the shortened stroke represents, and may represent a 50xc2x0 or more rotation of the output member, although the movement pattern may be adjusted to satisfy other requirements by means of changes in the coupling between the piston and the power output member and for example by cam profile changes. The relative reduction of stroke would typically be evident at the tail of the piston movement where cylinder pressure is low and torsional effort minimal. With relative reduction of the expansion stroke length, a similar relative reduction would also therefore apply to the stroke of the exhausting cycle. The duration of this stroke may remain at 90xc2x0 rotation of the output member. Alternatively, a reduced period may be required to match or comply with the combined dynamics of the exhaust and induction systems.
The relative reduction in rotation of the output member during the expansion and exhaust strokes permit a relative extension of the duration of the induction stoke, to enable a longer xe2x80x9cbreathing periodxe2x80x9d on the induction stroke.
The induction stroke may correspond to rotation of between 80xc2x0 and 150xc2x0 of the output member to facilitate induction of the charge, air, or fuel and air mixtures and to match the flow dynamics of inlet tract and valve flow characteristics, and hence provide better volumetric efficiency, while also avoiding the problems associated with valve overlap. The compression stroke length will be the same as the induction stroke length, but the output member rotation to execute the compression stroke is preferably less than 90xc2x0, and may be as little as 40xc2x0 rotation to provide a greater duration for the induction stroke, thereby enabling the combined kinematics of both strokes to be set for best pumping efficiency. The stroke length may also be shortened to permit changes of compression ratio.
Preferably, the piston speed will be held substantially constant or increasing during the last 25%-1% of the compression stroke, the specific piston kinematics being selected to suit particular fuels and operating cycles,. Ignition preferably takes place within the remaining 5% to 10% of the stroke before TDC. However, different fuels and operating conditions may require adjustment to the ignition setting to obtain ideal performance.
According to a further aspect of the present invention there is provided a method of operating a reciprocating piston machine in which a piston is connected to a rotating member and moves in one direction during a first induction stroke and in the opposite direction during a second compression stroke, and the degree of rotation of the rotating member is greater over said first stroke.
In use, the machine provides a longer duration on the induction phase and thereby improves the pumping efficiency of the machine.
According to a still further aspect of the present invention there is provided a method of operating a four-stroke reciprocating piston machine in which a piston is connected to a rotating member and moves in one direction during the first and third strokes and in the opposite direction during the second and fourth strokes, whereby the stroke length of the first induction stroke and the second compression stroke is greater than the stroke length of the third expansion stroke and the fourth exhaust stroke.
These aspects of the present invention may be used to advantage in the operation of compressors, pumps, and other machines, in addition to engines.